(1) Field of the Invention
The present invention relates to a cap for a writing instrument.
(2) Description of the Prior Art
Conventionally, as shown in FIGS. 1 to 3, there exists a cap for a writing instrument having a structure obtained such that a cap main body 9 is formed by injection molding using a plastic material so as to have upper and lower open ends, and a cap head 10 formed separately from the cap main body 9 and integrally having a seal cylinder 11 and clip 12 is mounted on the upper open end of the cap main body 9, as shown in FIG. 4.
This cap generally has undercuts 13 for engagement with the writing instrument main body at an inner surface portion near the lower open end of the writing instrument main body, as shown in FIG. 2. The undercuts 13 are often formed at a position e (see FIGS. 2 and 3) of the inner wall of the cap main body 9 which opposes a position on an axially extending line of a portion g (see FIGS. 1 and 2) facing the gate of a mold in injection molding, and arbitrary angular positions, e.g., every 90.degree., from the position e. The undercuts 13 are formed at these positions because drawing design and angular alignment in molding are facilitated.
In injection-molding a plastic using a mold, a weld line is formed on the resultant product. In the conventional cap main body 9, the weld line is formed at the same position of the inner wall as the position e opposing the position on an axially extending line of the portion g facing the gate of the mold. For this reason, the weld line often overlaps the undercut 13.
When the weld line is formed at the position of the undercut 13, and the cap main body 9 is repeatedly attached to or detached from the writing instrument main body, a crack may likely start at the undercut 13 of the cap main body. In particular, when a material such as PET which tends to degrade unless water content management in molding is sufficiently performed is used, this tendency becomes conspicuous. When cracking occurs, the crack becomes noticeable in a transparent material such as PET, although the fitting failure between the cap main body 9 and the writing instrument main body does not occur. Consequently, the product value decreases, and the user feels uncomfortable to use the pen.